Electrostatic clutch

ABSTRACT

An electrostatic clutch is described comprising a plurality of micron-scale thickness electrodes, adjacent electrodes being separated by a thin film of dielectric material. A power source and controller apply a voltage across two electrodes, causing an electrostatic force to develop. When engaged, a force can be transferred through the clutch. A tensioning device maintains the alignment of the clutch when the electrodes are disengaged, but permits movement in at least one direction. In some embodiments, multiple clutches are connected to an output to provide variable force control and a broad range of torque input and output values. Moreover, the clutch can be used as an energy-recycling actuator that captures mechanical energy from negative work movements, and returns energy during positive work movements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §§ 119-120 ofProvisional Ser. No. 62/495,693, filed Sep. 21, 2016 and PCT ApplicationNo. PCT/US2015/055005, filed Oct. 9, 2015, which claims priority toProvisional Ser. No. 62/122,066, filed Oct. 9, 2014, and ProvisionalSer. No. 62/231,818, filed Jul. 16, 2015, all of which are incorporatedherein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under NSF Grant No.IIS-1355716. The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

The invention relates generally to an electrostatic clutch. Morespecifically, the invention relates to a lightweight and high powerdensity electrostatic clutch that can be incorporated into roboticsystems, including exoskeletons and wearable devices, among other uses.

Clutches have many uses in mechanical systems, often being used toimprove the functionality of springs and actuators. However, existingclutch systems suffer several drawbacks when used in mobileapplications, such as robotics. For example, electromagnetic clutchesfeature fast activation and moderate torque density, but requirecontinuous electrical power to stay active. Magnetorheological clutchesproduce large torques, but are heavy and also require continuous powerto remain active. Because of the power requirements, both of thesesystems require large batteries or tethered electrical connections.Mechanical latches require no energy to stay active, but only engage anddisengage under special conditions.

The problems associated with traditional clutches are particularlypronounced in wearable robotic systems, such as exoskeletons. Assistiverobotic exoskeletons have shown positive impacts for people in a varietyof applications, including physical performance augmentation and medicaltreatment. One challenge associated with autonomy is the metabolic costassociated with carrying the combined weight of the exoskeletonstructure, energy storage, actuators, and electronics. Batteries inparticular account for a significant portion of the weight of manydevices, especially in devices with clutches that require constantpower. In addition to the weight of batteries, significant weightpenalties are experienced with commercially available actuators, such asmotors and pneumatic actuators.

Walking on level ground is an example of an application wheretraditional actuators and motors are not well suited for roboticapplications. Walking on level ground at a constant speed requires verylittle energy input since the potential and kinetic energies of themoving body do not change on average. However, approximately equalamounts of positive and negative work are performed by the legs during awalking cycle. Both the positive and negative work require energy, sincethe negative work cannot be stored and reused as an input for thepositive work.

If an actuator could absorb and return mechanical energy, the totalenergy consumption of the system could be reduced. Ideally, energyrecycling could supply all needed positive work by absorbing and reusingnegative work movements. As an added benefit, using a device to absorbenergy from negative work movements would reduce the metabolic cost of ahuman wearing a robotic device because muscles require energy to performnegative work.

Lightweight, low-power, and electrically controllable clutches wouldallow greater performance of many robotic systems. Improved clutchescould be incorporated into actuators to substantially improve theactuator's energy demands. Therefore a need exists for a clutch thatdoes not exhibit any of the shortcomings of traditional clutches.

BRIEF SUMMARY OF THE INVENTION

According to embodiments of the present disclosure is an electrostaticclutch that can be incorporated in many types of mechanical systems,such as robotics, wearable devices, or exoskeletons. In particular, thepresent invention utilizes micron-thickness electrostatic clutches thatare light-weight and consume minimal power.

Electrostatic forces can be developed by applying a voltage to a set ofelectrodes separated by a gap. In the present invention, the gap ismaintained by a layer of dielectric material deposited on the electrode.When a voltage is applied, positive and negative electrical chargesdevelop, causing an attraction between the adjacent electrodes andpreventing them from moving relative to each other. Like a capacitor,power consumption is very low once a charge is developed becauseadditional energy is only required when switching states. A controllercan manipulate the voltage, allowing electrical ‘on-off’ control ofadhesion between the electrodes.

The electrodes comprise a lightweight conductive material, such asaluminum-sputtered biaxially-oriented polyethylene terephthalate. With apair of electrodes, at least one electrode is covered in a dielectricmaterial to maintain the gap between the conductive surfaces of theelectrodes. In some embodiments, the electrodes are generally planar,having a rectangular or square shape. A frame is connected to each ofthe electrodes, providing a transfer point for a force acting on theclutch. For example, the frame of one electrode could be connected to aspring, while the frame of the other electrode could be connected to thebody of an exoskeleton. Thus, the activation state of the clutchdetermines if a force is transferred from the spring to the body of theexoskeleton through the clutch, or if the electrodes will simply slideagainst each other without transferring the force. A tensioner maintainsalignment of the electrodes, while permitting movement in one ormultiple directions.

In an alternative example, three electrodes are arranged in a parallelorientation. One electrode is attached to a body of a device and asecond electrode is connected to an output supplying a force. A thirdelectrode is connected to a spring and placed between the first andsecond electrodes. The electrode connected to the spring can be engagedagainst either the body electrode or the output electrode. In thisconfiguration, the spring can be stretched by the output force, affixedto the frame to store the energy, then later returned to the output toperform work, forming a type of energy recycling actuator. Given themicron-scale thickness of the electrodes, the actuator can be comprisedof tens to hundreds of clutch/spring pairs that are individually engagedand disengaged with the output, thus allowing variable stiffness and abroad range of torque input and output values over the course of oneactuator stroke.

The clutch system of the present application, depending on theparticular implementation, uniquely allows for both force control andenergy recycling, making it both highly controllable and highly energyefficient. In addition, this system allows variable stiffness, impedanceor other state-dependent force generation at exceptionally highbandwidth and with low input of control energy. The clutch system willenable dramatic improvements in the energy efficiency andcontrollability of autonomous robotic systems and wearable roboticdevices.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a clutch according to an embodiment of the presentinvention.

FIG. 2 shows two electrodes that are used in the clutch.

FIG. 3 is an alternate view of the electrodes, which also shows a forcetransferring spring attached to one of the electrodes.

FIGS. 4A-4B is a schematic showing the components of the electrodes,according to one embodiment.

FIG. 5 is a diagram showing the electrical components of a power sourceand controller attached to the electrodes.

FIG. 6 shows an energy-recycling actuator comprised of the clutches ofthe present invention.

FIGS. 7A-7D is an alternate embodiment of the energy-recycling actuatorshowing the stages of operation.

FIG. 8 is a graph illustrating the variable force profile of a deviceincorporating a plurality of clutches.

FIG. 9 is an alternate embodiment incorporating an embodiment of theclutch of the present invention.

FIG. 10 is a graph showing the ‘on-off’ control of clutches used in anexoskeleton during a walking cycle.

FIGS. 11A-11B are views of a device incorporating electrostaticclutches, according to one embodiment of the present invention.

FIG. 12 is yet another device incorporating electrostatic clutches.

FIG. 13 is a clutch that can be used with a rotating shaft.

FIGS. 14A-14C show multiple clutches used in a device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention and its advantages are bestunderstood by referring to the figures. FIG. 1 shows the clutch 100 ofthe present invention according to one example embodiment. As shown inFIG. 1, the clutch 100 is comprised of a first electrode 101 and asecond electrode 102. The electrodes 101, 102 are aligned in a parallelorientation so that a surface of the first electrode 101 overlaps asurface of the second electrode 102. The conductive surfaces 202 do notcontact as they are separated by a layer of dielectric material 203deposited on one or both of the electrodes 101, 102. In this exampleembodiment, only one electrode 101, 102 is coated with a layer ofdielectric material 203 to minimize the distance between the electrodes101, 102.

A frame 103 is attached to one end of the first electrode 101 and aseparate frame 104 is attached to one end of the second electrode 102.The frames 103, 104 are positioned at opposite ends of the clutch 100,as shown in FIG. 1. The frames 103, 104 provide a point of transfer fora force acting on the clutch 100. For example, in the embodiment shownin FIG. 1, the bottom frame 104 is attached to a flat rubber spring 301.The spring 301 can be attached to an object such as the body of anexoskeleton, for example. If the clutch 100 is in an engaged state—whenan electrostatic force is causing an attraction between electrodes 101,102—the spring 301 will impart a tensile force through the electrodes101, 102 to the top frame 103. When disengaged, the force caused by thespring 301 will simply result in the bottom electrodes 102 slidingfreely against the top electrode 101. In other words, the force will notbe transferred to the top frame 103.

Referring again to FIG. 1, a tensioner 105 connects the first electrode101 to the second electrode 102. The tensioner 105 maintains thealignment of the electrodes 101, 102 so that the surfaces of each are inproximate engagement, while also permitting linear movement of theelectrodes 101, 102 along the vector of an outside force acting one ofthe frames 103, 104, when the clutch 100 is in a disengaged state. Inthe preferred embodiment, the tensioner 105 comprises an elastic cord106 or low-stiffness spring connecting a distal end of the firstelectrode 101 to a proximate end of the second electrode 102. Thetensioner 105 further comprises an additional cord 106 connecting theproximate end of the first electrode 101 to the distal end of the secondelectrode 102. A cord 106 is provided on each side of the electrodes101, 102 to provide lateral stability. In this configuration,side-to-side movement of the electrodes 101, 102 is suppressed, whereasup-and-down movement is allowed. Further, movement orthogonal to thesurface of the electrodes 101, 102 is minimized, keeping them in closecontact. In alternate embodiments, the tensioner 105 can comprise a pairof frames, each frame attached to the edge of one of the electrodes 101,102. In this alternative embodiment, the frames are held in place by atrack that permits each frame to slide up-or-down, but not laterally.

FIG. 2 shows an alternate view of the clutch 100 according to theembodiment shown in FIG. 1. In FIG. 2, the electrodes 101, 102 areseparated from each other to show detail. In operation, the electrodes101, 102 would be stacked on top of each other, causing close engagementof the surface of each. As shown in FIG. 2, the cords 106 attach to abar 107 affixed to the ends of the electrodes 101, 102. FIG. 3 is yetanother view of the electrodes 101, 102 of the clutch 100. In FIG. 3, aflexible lead 108 is run along the spring 301, providing an electricalcontact for the electrode 102.

FIGS. 4A-4B show in greater detail the construction of the electrodes101, 102 according to one embodiment. As shown in FIG. 4A, firstelectrode 101 and second electrode 102 are comprised of a substrate 201onto which a conductive layer 202 is deposited. In applications whereflexibility of the clutch 100 is desired, the substrate 201 can beconstructed from a flexible polymer sheet. For example, in oneembodiment, the electrode 101 is comprised of aluminum-sputtered BOPET(Bi-axially Oriented Polyethylene Terephthalate) film, also known asMylar® film. The aluminum deposition acts as the conductive layer 103and the BOPET acts as the substrate 102. Aluminum-sputtered BOPET filmsof this type can have a thickness of around 25 microns. Despite the thinprofile, the material is sufficiently strong to act as a forcetransmission component. In addition, very little electrode material isrequired to hold a charge, making thin and lightweight electrodes 101possible. In alternative embodiments, a single-layer, conductiveelectrode, such as a metallic foil, is used.

FIG. 4A shows the electrodes 101, 102 in a disengaged state. That is, noelectrostatic charge is present, so the electrodes 101, 102 are notattracted to each other. (The space between the electrodes 101, 102 inFIG. 4A is exaggerated for purposes of illustration.) When disengaged,the electrodes are free to move along an axis, as indicated by thearrows in FIG. 4A. A tensioner 105, if present, prevents the electrodes101, 102 from moving in other directions. However, in someimplementations, it may be desirable to have the electrodes 101, 102move in more than one direction, while still maintaining their parallel,or substantially co-planar, arrangement.

In contrast, FIG. 4B shows the electrodes 101, 102 in the engaged state.In the engaged state, a voltage supplied by power source 401 creates anelectrostatic charge, causing an attraction of the electrodes 101, 102and drawing the surfaces of electrodes 101, 102 together. Once engaged,the electrodes 101, 102 can then be loaded in shear, and the frictionforce resulting from the electrostatic normal force prevents relativedisplacement of the electrodes 101, 102.

As previously stated, the electrodes 101, 102 can be flexible accordingto some embodiments. The compliant nature allows intimate surfacecontact between the electrodes 101, 102 when engaged. This allows thesurfaces to conform closely without relying on a high surface energyinterface. In previous electrostatic devices, electrodes are embedded insoft, tacky elastomers. Releasing these types of devices requires aseparate mechanism because the elastomers tend to stick to each otherafter being drawn together by the electrostatic forces.

When in the engaged state, a layer of dielectric material 203 maintainsthe gap between the conductive layers 103 on adjacent electrodes 101,102. In one example embodiment, a thin film of dielectric material 203is disposed on the surface of one of the electrodes 101, 102, coveringthe conductive layer 202. At a given voltage, the capacitance of theclutch 100 increases as the dielectric constant of the material used forthe insulating layer 203 increases. As such, a high dielectric constantmaterial is desirable to allow operation at a relatively low voltage.However, the type of dielectric constant material used can depend on theparticular application. The dielectric layer 203 can be an inorganicparticle impregnated polymer or a liquid-formable nanoparticlecomposite. In one example, a ceramic polymer composite containing bariumtitanate and titanium dioxide is used to create the dielectric layer203. An example of such a material is Dupont™ LuxPrint® material, whichis sold for electroluminescent applications. With a low voltage, 200-300V for example, standard electronics hardware can be used with the clutch100.

Testing of a liquid formable nanoparticle composite indicates thecapability to produce 6 times higher pressures at 15 times lower fieldstrengths than the inorganic polymer (Table 1).

TABLE 1 Measured electrostatic clutching properties of dielectricmaterials Coefficient Max. Observed Min. of Static Relative ShearPressure Release Material Friction Permittivity (Field Strength) TimeInorganic 0.34 ± 0.04 1.43 ± 0.09 1.82 ± 0.11 kPa — particle (38 MV/m)impregnated polymer Liquid 0.40 ± 0.04 10.1 11.6 kPa 6 ms formable (2.5MV/m) nanoparticle composite

Increasing field strength and voltage provide diminishing shear pressureafter a critical value is reached. This result is attributed to thedevelopment of space charge. Space charge occurs when charge carriersare forced into the dielectric material 203 from the electrodes 101, 102and become trapped. This creates an internal electric field thatcounteracts the applied field, and produces some force even after theelectrodes 101, 102 are grounded, causing slow or no release. Thiseffect is dependent on the chemical makeup of the dielectric material203. Avoiding space charge is critical to achieving effective pressuredevelopment and fast releasing.

The detrimental effects can be reduced by maintaining low electric fieldstrength and voltage. Consequently, decreasing the thickness andsubsequently the overall voltage value can mitigate space chargeeffects, but the electric field strengths should also be kept low. Theliquid formable nanoparticle composite is used in the example embodimentbecause the high dielectric of the material reduces required fieldstrengths. Also, because the liquid formable nanoparticle composite isobtained in its uncured form, it can be incorporated onto the clutch 100with a lower thickness.

By way of example, the process of applying the dielectric layer caninclude depositing a 25 micron layer of the liquid formable nanoparticlecomposite on one side of electrode 101 using a thin film applicator.Based on the particular dielectric material 203 used, the composite iscured to a thickness of 10 microns in a ventilated oven. A second 25micron layer is then applied and cured to a final dielectric layer 203of 20 microns. The film decrease in thickness occurs because asignificant amount of solvent evaporates from the original mixtureduring curing. Other methods can be used to deposit the dielectric layer203, such as screen printing or chemical and physical deposition.

FIG. 5 illustrates one example of a power source 401 and controller 402capable of controlling the clutch 101. As shown in FIG. 5, ahigh-voltage power supply 403 (240V, for example), supplies a voltage tothe circuitry of the power source 401. A control voltage fromcontrollers 402 are fed into a transistor 404. The controller 402 can beany device capable of producing a signal. In this particular embodiment,the transistor 404 is a Darlington pair transistor. The transistors 404are connected to a high-voltage relay 405 powered at 1.9V by an externalpower supply. The relays 405, in turn, are connected to the conductivesurface 202 of the dielectric coated electrode 101, such that theelectrode 101 is either at high voltage, at ground voltage, or floating.The second electrode 102 is connected to ground. While one particularpower source 401 example has been described, a person having skill inthe art will appreciate that many types of electrical configurations canbe used to apply a voltage to the electrodes 101, 102 of the clutch 100.

With potentials as low as 200 V between electrodes 101, 102, a shearpressures of 15 kPa is generated across electrodes 101, 102 described inthe embodiments shown in FIGS. 1-2. In addition, the electrodes 101, 102fully release in less than 6 ms upon switching of voltage. This resultsin clutches 100 that have 100 times lower mass and energy use thantraditional clutches and 100 times lower voltage and faster release timethan otherwise comparable electrostatic clutches.

As a result, when implemented in an actuator or other device, thelow-mass, low-energy, and low-volume electrostatic clutch 100 of thepresent invention allows multiple clutches 100 to be used in a singledevice. Because of the unique geometry of these electrostatic clutches100, many can be “stacked” into a small volume with a spacing of 1 mm orless between clutches 100. Achieving tens or hundreds of clutches in adevice using traditional mechanical or electromagnetic clutches resultsin a slow, energy-expensive device far too large and heavy to bebody-mounted.

By way of example of a system thus described, a stacked clutchimplementation can comprise 5 electrode pairs, each having a thicknessof 45 microns and a mass of 2 grams. The contact area of the pairs is100 cm² (10 cm×10 cm), resulting in a holding force of 150 N. Theswitching energy required to change from an engaged to disengaged stateis 0.01 J. Switching can occur at a bandwidth of 160 Hz.

The clutch 100 of the present invention is designed to be generic enoughto be “attach-and-play” on assistive exoskeletons, active prostheses,walking robots, and other devices. It is an aspect of the presentinvention that the design can be modified for use with a motor as avariable stiffness actuator, or to achieve “one-to-many” degrees offreedom by decoupling an input from an output. This can be achieved byadding single clutch 100 in series with a clutch-spring pair.

In another embodiment, multiple electrodes are arranged in parallel tocreate a type of energy-recycling actuator 500, which is illustrated inFIG. 6. As shown in FIG. 6, the actuator 500 comprises a housingelectrode 501 that is connected to a device housing 502. A springelectrode 503 is connected to a spring 301 (or other energy storingdevice) and is positioned adjacent to the housing electrode 501.Further, the spring electrode 503 is coated with a dielectric layer 203on two surfaces. A third electrode 504 is placed on the other side ofthe spring electrode 503, so that the spring electrode 503 is in themiddle of the housing electrode 501 and the third electrode 504. Thethird electrode 504 is connected to an object not connected to thehousing 502. When a voltage is applied across two of the threeelectrodes, it causes those two electrodes to adhere, preventing lateralmovement.

In operation, selective engagement of the electrodes 501, 503, 504 canresult in an energy recycling cycle. An example of an energy recyclingcycle for a similar actuator 500 is illustrated in FIGS. 7A-7D. As shownin FIG. 7A, during the first step, the third electrode 504 and thespring electrode 503 are engaged, causing the spring 301 to stretch andstore energy as a force acts on the object and third electrode 504.Next, in FIG. 7B, the third electrode 504 and spring electrode 503 aredisengaged, followed immediately by the spring electrode 503 engagingthe housing electrode 501. This allows the housing 502 and object tomove freely with respect to each other while the spring remainsstretched indefinitely. Next, as shown in FIG. 7C, the spring electrode503 disengages from the housing electrode 501 and re-engages with thethird electrode 504, connecting the spring 301 to the object, providingmechanical work to assist the motion of the object. In FIG. 7D, thespring electrode 503 is engaged to the housing electrode 501, allowingfree movement of the object relative to the housing 502 withoutstretching the spring 301.

Energy capture and return could be achieved with a single spring 301permanently engaged with the object. However, assistance timing andperiods of non-interference are important for many tasks. A devicecapable of periodically allowing free movement therefore offers moreutility. The actuator 500, using a spring 301 and a three-way clutchingmechanism provides this utility. In the example of an exoskeleton, thespring 301 could absorb energy from a human's negative work movement. Atthe end of the movement, the spring 301 is engaged to the housing 502,storing the energy and allowing free movement of the object. When energyis required to assist in a positive work movement, the third electrode504 is re-engaged with the spring 301, which shortens as it returnsenergy to the human.

The foregoing example describes an actuator 500 comprised of a singlespring/clutch mechanism. However, multiple spring/clutch pairs can beused to create an actuator capable of providing variable stiffness. Thatis, if all springs 301 are engaged, actuator 500 will have a highstiffness. If only a fraction of the springs 301 are engaged, while theremainder are disengaged and free to move, the actuator 500 will have areduced stiffness. Consequently, the stiffness of the actuator can bemanipulated based on the appropriate level for different types ofactivities. With a higher stiffness, higher assistive torques will beprovided.

By engaging increasing or decreasing numbers of springs 301 during anactuator stroke, a variety of force values can be achieved, independentof device configuration. For example, FIG. 8 shows theforce-displacement curve for five clutched springs 301 in parallel.Placing multiple clutched springs 301 in parallel allows an overalldevice stiffness to be selected. The maximum device stiffness is 36times higher than the minimum device stiffness in this example.

FIG. 9 shows the electrostatic clutch 100 of the present inventionincorporated in to an assistive exoskeleton. In this embodiment, anelastomer spring 301, such as a natural rubber or urethane sheet, isattached to frame 104. Elastomer springs 301 have two significantadvantages: they are composed of material with high strain energydensity, and they allow an axial loading configuration which furtherimproves energy density because all material is strained equally. Inthis embodiment, each spring 301 has a mass of about 5 grams, resultingin a total spring mass of about 25 grams for a five spring 301configuration. The resilience, or efficiency, under normal walking andrunning conditions is about 95%.

The opposite end of the spring 301 is attached to a lower portion 603 ofan exoskeleton frame 601. The exoskeleton frame 601 is a lightweight,high-strength composite frame having a hinge 602 at the ankle,connecting the lower portion 603 to an upper portion 604. Frame 103 isconnected to the upper portion 604 of exoskeleton frame 601. During awalking cycle, flexing of the foot causes stretching of the spring 301when the electrodes 101, 102 are engaged. The energy of the spring 301can be released during other phases of the walking cycle.

FIG. 10 shows the clutch 100 activation and deactivation phases during asingle step. First, the clutch 100 engages at maximum dorsiflexion, asthe foot hits the ground at the beginning of the step. At this point,the spring 301 is slack and not storing energy. As the step continuesand the angle of the angle decreases, the spring 301 is stretched. Ifthe electrodes 101, 102 were not engaged, the force on the spring 301would simply cause the electrodes 101, 102 to slide against each other.The stretching absorbs some of the negative work that would otherwise beperformed by human muscles. After peak stretch, the spring 301 providesenergy as the foot begins to push off the ground, increasing the ankleangle and shortening the spring 301. As the foot leaves the ground, theclutch 100 deactivates, allowing free rotation of the ankle prior to thecycle starting over. Without the free rotation, energy would have to beused to stretch the spring 301 as the toe is lifted prior to the foothitting the ground for the next step.

In the preferred embodiment, the electrode 101 is switched between highvoltage and ground at 200 Hz for 50 ms to facilitate clutch release. Inthis example, peak torque is about 7.3 N*m on an average step, and thedevice consumes about 8.7 mW of electricity.

FIGS. 11A-11B depict the electrostatic clutch incorporated into atransmission belt assembly 700, where the clutch selectively couples aninput shaft 711 and an output shaft 712. In this embodiment, atransmission belt 710 comprises one of the electrodes 101 or 102 of theclutch 100. The second electrode of the clutch 100 is disposed on thesurface (or comprises the surface) of the shafts 711 and 712. As aresult, when no voltage is applied across the electrodes 101 and 102(i.e. the clutch is in the disengaged state), the clutch belt 710 willslip at the shafts 711 and 712. When the clutch is engaged, the belt 710adheres to both shafts 711 and 712, and torque is transmitted from theinput shaft 711 to the output shaft 712. In an alternative embodiment,the input 711 and output 712 shafts can be aligned with concentricelectrodes 101, and 102 dispersed between the two shafts 711 and 712. Inthis embodiment, the clutch 100 can replace the functionality of rotaryelectromagnetic clutches and magneto-rheological clutches, while usingsignificantly less electrical power. Further, when used in thisembodiment, the pretension of the transmission belt 710 can beminimized.

In this example shown in FIG. 11B, a type of variable transmission canbe created by providing different sections 721, 722, and 723 on theoutput shaft 712. The sections 721, 722, and 723 can be connected togears with different ratios. For example, section 721 can be connectedto a gear with a first ratio, section 722 can be connected to a gearwith a second ratio, and section 723 can be connected to a gear with athird ratio. By selectively applying a voltage to one of the sections721, 722, or 723, the transmission belt 710 will transmit the torquethrough a section with the desired gear ratio.

FIG. 12 depicts another embodiment incorporating the clutch 100 of thepresent invention. In this embodiment, a plurality of output shafts 712are connected by a single transmission belt 710, which is driven by aninput shaft 711. The voltage at each output shaft 712 can be controlledto selectively engage or disengage each individual output shaft 712.

FIG. 13 depicts the clutch 100 in an alternative embodiment, with thefirst electrode 101 comprising a rigid shaft and the second electrode102 comprising a flexible electrode with rigid bars 801. The rigidelectrode 101 can be connected to a motor. The bars 801, which can beconstructed from carbon fiber in one example embodiment, can then beconnected to an output. When the clutch 100 is engaged, the rigid bars801 will turn with the shaft. In the disengaged state, the shaft willspin freely without turning the second electrode 102 and rigid bars 801.

FIGS. 14A-14C show this type of clutch 100 with a plurality of clutches100 disposed around a plurality of rigid shafts. The shafts areconnected to a common input plate connector 802. The rigid bars 801, inturn, will be connected to an output connector 803. Thus, when theclutches are in the engaged state, torque from the input connector 802will be transmitted to the output connector 803 through the rigid bars801. The use of concentric clutches allows an increase in the amount oftorque that can be transmitted in the same volume.

While the disclosure has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modification can be made therein withoutdeparting from the spirit and scope of the embodiments. Thus, it isintended that the present disclosure cover the modifications andvariations of this disclosure provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. An electrostatic clutch for use in a roboticsystem, the clutch comprising: a first electrode comprising a conductivefilm, wherein a surface of the first electrode is coated with adielectric material to cover the conductive film; a first frame affixedto an end of the first electrode; a second electrode comprising aconductive film, wherein a surface of the second electrode is alignedparallel to the surface of the first electrode, wherein the dielectricmaterial separates the first electrode surface from the second electrodesurface; a second frame affixed to an end of the second electrode; atensioner connecting the first electrode to the second electrode,wherein the tensioner maintains an alignment of the first electrode andthe second electrode, while allowing relative movement of the first andsecond electrodes in at least one direction parallel to the firstelectrode surface; and a power source for applying an electric fieldacross the first electrode and the second electrode to develop anelectrostatic charge, causing the first electrode and the secondelectrode to exist in either an attractive state or a non-attractivestate, wherein the tensioner allows the first electrode and the secondelectrode to move linearly relative to each other when the firstelectrode and the second electrode are in the non-attractive state,wherein the first frame and the second frame are coupled through thefirst electrode and the second electrode when in the attractive state.2. The electrostatic clutch of claim 1, wherein the dielectric materialis comprised of a polymer composite containing barium titanate andtitanium dioxide.
 3. The electrostatic clutch of claim 1, wherein thefirst electrode and the second electrode comprise a flexible substratecoated with a conductive layer.
 4. The electrostatic clutch of claim 3,wherein the substrate is a polymer.
 5. The electrostatic clutch of claim3: wherein the substrate is bi-axially oriented polyethyleneterephthalate, wherein the conductive layer is sputter-depositedaluminum.
 6. The electrostatic clutch of claim 1, wherein the tensionerfurther comprises: an first elastic cord connecting the first frame tothe second electrode, and a second elastic cord connecting the secondframe to the first electrode.
 7. The electrostatic clutch of claim 6,wherein the tensioner further comprises: a first bar attached to thefirst electrode opposite the first frame, wherein the elastic cordattaches to the second frame and the first bar; and a second barattached to the second electrode opposite the second frame, wherein theelastic cord attaches to the first frame and the second bar.
 8. Theelectrostatic clutch of claim 1, further comprising: a plurality ofclutches connected to an output; a controller electrically connected toeach of the plurality of electrostatic clutches, wherein each clutch ofthe plurality of electrostatic clutches can be engaged or disengaged toprovide a variable torque on the output.
 9. The electrostatic clutch ofclaim 1, further comprising: a third electrode comprising a conductivefilm, wherein a surface of the third electrode is aligned parallel tothe surface of the first electrode and the surface of the secondelectrode, wherein the third electrode is positioned between the firstelectrode and the second electrode; wherein the dielectric materialseparates the third electrode surface from the first electrode surfaceand the second electrode surface; a housing, wherein the first electrodeis attached to the housing; and a spring connected to third electrodeand attached to the housing, wherein the third electrode can engage thefirst electrode to store energy in the spring, wherein the thirdelectrode can engage the second electrode to transfer energy from thespring to the second electrode.
 10. The electrostatic clutch of claim 1,further comprising: a spring having a first end and a second end, thefirst end connected to the second frame; and an exoskeleton frameadapted to be worn on a lower leg to assist with walking, theexoskeleton frame comprising: a lower portion connected to the secondend of the spring; an upper portion connected to the first frame; and ahinge connecting the lower portion and the upper portion, wherein thehinge permits movement of the lower portion relative to the upperportion to stretch the spring when the first electrode and the secondelectrode are in the engaged state.